The present invention relates to an information recording medium, and more particularly, to an optical information recording medium, which is capable of reading out or recording with a high recording density, and which has a high reliability in repeating recording and regeneration operations.
Conventionally, compact disks (CD), laser disks (LD), and the like are used widely as optical information recording media. Currently, a DVD, which has seven times the recording density of a CD, has come into practical use. The DVD is being developed as an erasable recording-regenerating medium in addition to a read only medium (DVD-ROM), wherein information is directly written onto the substrate. Furthermore, the practical use of a DVD as a RAM for a computer presently is under investigation.
With the DVD, high density recording can be achieved by using a laser having a shorter wave length, such as 650 nm, than the laser used for a CD (wave length approximately 780 nm). However, in order to handle a large amount of information, such as computer graphics and the like, it is necessary to achieve a higher recording density, such as 1.5 to 2 times that of the conventional high density recording. In order to achieve such a high recording density, a semiconductor laser of green to blue color having a shorter wave length (wave length 520-410 nm) than ever is under development.
As another means to achieve a higher recording density, a super resolution film can be employed. The super resolution film is a thin film formed at a lower plane of the recording medium, with which a high recording density can be achieved by the fact that it is able to decrease the size of the beam spot of the incident light passing through the film.
One of the mechanisms of the super resolution effect is an absorption-saturation phenomenon, which is a phenomenon utilizing non-linear optical characteristics of the super resolution film such that the film allows light having a larger intensity than the amount of its absorption-saturation to pass through the film and absorbs any light having an intensity less than the amount of its absorption-saturation. The spatial intensity of a laser beam utilized in reading and writing has a Gaussian distribution. Therefore, when the laser light beam passes through the super resolution film, the laser light in the lower end portion of the Gaussian distribution, where the intensity is low, is absorbed by the film, and the laser light in the middle portion of the Gaussian distribution, where the intensity is high, passes through the film. Accordingly, the diameter of the laser beam is reduced as it passes through the super resolution film.
An organic thin film made of a material in the phthalocyanine group, as disclosed in JP-A-8-96412 (1996), chalcogenide, fine particles of a compound semiconductor, and the like are known at the present as materials which may be used for the super resolution film described above. Additionally, trials to use some organic materials, such as thermochromic materials of the type disclosed in JP-A-6-162564 (1994), and photochromic materials of the type disclosed in JP-A-6-267078 (1994), as the super resolution film have been carried out.
However, the above-mentioned materials have problems in reliability and productivity. That is, there has been a concern about gradual deterioration of the organic thin film after repeated recording and regenerating operations, because the energy density of a laser beam is locally increased significantly during the recording and regenerating operations. Therefore, a sufficient guarantee period for the recording and regenerating operations is scarcely obtained under a severe condition of use, wherein the recording and regenerating operations are performed frequently, such as when the disk is used as a RAM and the like for computers.
On the other hand, chalcogenide is chemically unstable, and so a long guarantee period can not be obtained for this material, and the fine particles of a compound semiconductor provide difficulties during the production process.
One of the objects of the present invention is to provide an optical recording medium having a super resolution film, which can guarantee repeated recording and regenerating operations for a sufficiently long time, and which has a preferable productivity and a high resolution effect.
A first aspect of the present invention to solve the above issues is an optical information recording medium comprising a substrate, whereon a recording layer for recording information is formed; and a glass thin film, formed onto the substrate, having a characteristics such that the intensity distributions of irradiated light onto the glass and transmitted light through the glass vary in a non-linear manner.
The substrate is desirably transparent to light, and for instance, is made of inorganic materials, such as glass and the like, and organic materials, such as polycarbonate, polyethylene terephthalate, and the like are also desirable. Here, the term glass refers to amorphous solid oxides and general amorphous materials containing the above oxide as a main component.
Forming on a substrate includes both forming onto the surface of a substrate directly and forming onto the surface of a substrate indirectly via another layer, for instance, a protection layer.
In accordance with the above composition, an information recording disk, which has a large capacity, and which experiences less deterioration after repeated reading out and writing, can be provided.
In the first aspect of the invention, the recording layer can be provided with a pit pattern representing the recording information. The pit pattern is a device by which the information is recorded in accordance with the arrangement of pits provided onto the surface of the substrate. If this recording method is employed, the recorded information can not be rewritten. However, once a master die of the substrate having this recorded information is made, a large number of substrates with the same information can be manufactured readily. Therefore, this recording method is used for recording movies, music, and computer programs.
The recording layer of the invention can also be a device for recording information with optical energy. For recording information with optical energy, an information recording substrate using so-called phase changing organic materials or inorganic materials, the crystalline structure of which varies when irradiated by light, is used as the recording layer.
A second aspect of the present invention is an optical information recording medium comprising at least a substrate, a recording layer for recording information formed o n the substrate, and a reflecting film for reflecting light formed on the recording layer, wherein the substrate is made of glass, the optical transmittance of which increases in a non-linear manner corresponding to a n increase in intensity of the irradiated light.
In accordance with the above composition, a reflection type information recording disk, which has a large capacity and less deterioration after repeated reading out and writing, can be provided.
This second aspect of the invention provides an information recording substrate of a type, which reflects incident light with a reflecting film provided at a lower portion of the recording film, and reads the information with reflected light.
The glass in the first or the second aspects of the invention desirably contains at least an element selected from transition metal elements and rare earth metal elements.
For the above transition metal elements and the rare earth metal elements, particularly, at least an element selected from the group consisting of Tiy, V, Cr, Mn, Fe, Co, Ni, Nd, Ce, Pr, Sm, Eu, Tb, Ho, Er, and Tm is desirable.
When the transition metal element or the rare earth metal element forms a glass film, the metal element is desirably contained in the range from 20% by weight to 90% by weight as an oxide to the total weight of the glass. When the metal element forms a glass substrate, the metal element is desirably contained in the range from 0.1% by weight to 29% by weight as an oxide to the total weight of the glass.
In the first aspect of the invention, the glass desirably contains as oxide the following compounds: SiO2: 6-80% by weight, R2O: 0-20% by weight (copyright)=alkali metal element), B2O3: 0-30% by weight, and CoO: 20-90% by weight.
In the second aspect of the invention, the glass desirably contains cobalt oxide as CoO in the range of 0.1-29% by weight.
A third aspect of the present invention is an information recording medium comprising at least a substrate, whereon a recording layer for recording information is formed, and a super resolution layer formed on the substrate, the optical transmittance of which increases in a non-linear manner corresponding to an increase in the intensity of the irradiated light, wherein an output maintaining rate of the information recording medium after repeating the recording by 104 times is at least 90%.
The output maintaining rate is a value indicating how much of the intensity of the electrical signal is maintained after repeating the recording and regeneration by 104 times, taking the intensity of the electrical signal at the first regeneration of information after performing the first recording with irradiation of light as 100%. If the super resolution film is deteriorated by repeating the irradiation of light, the spot size of the laser ray which reaches the recording layer is expanded, and, as a result, the electric output is decreased. That means that a super resolution film which can maintain the initial output maintaining rate as long as possible is desirable.
Furthermore, in accordance with a fourth aspect of the present invention, an information recording medium is provided, which comprises a transparent substrate, and a recording layer for recording information which is formed onto the substrate, wherein an output decrease in recorded signal at a frequency of 8 MHz is less than xe2x88x9230 dB of the output at 1 kHz, and an output maintaining rate after repeating the recording by 104 times is at least 90%.
FIG. 8 is a graph indicating a relationship between the recording frequency and the output for the information recording media with and without the super resolution film of the present invention. The medium with the super resolution film can record signals of higher frequency components, because the spot size of the laser beam reaching the recording layer is decreased. The above composition indicates an index which represents how high a frequency component can be recorded.
In accordance with a fifth aspect of the present invention, glass comprising SiO2: 6-80% by weight, R2O: 0-20% by weight (copyright)=an alkali metal element), B2O3: 0-30% by weight, CoO: 20-90% by weight, as equivalent oxide, respectively, is provided.
The above glass can be mounted not only on a photo disk, but also on various media, as a film having the super resolution effect. For instance, a display apparatus, which generates light when its fluorescent body is irradiated with a laser ray so as to be excited, can produce a high resolution display by mounting the grass film of the present invention onto a surface of the fluorescent body, because the spot size of the laser ray can be converged.
In accordance with a sixth aspect of the present invention, a glass thin film containing cobalt oxide in the range of 20-90% by weight as equivalent CoO is provided.
In the case of this glass film, the upper limit of the CoO content is restricted, because, if CoO is added excessively, the CoO is precipitated, and causes devitrification.